The Surveillance Group had denied planting a hidden microphone in the Ecuadorian embassy in London.

ABC News: Patrick Rocca

A British surveillance company has denied accusations by the Ecuadoran government that it planted a hidden microphone in Quito's embassy in London, where WikiLeaks founder Julian Assange is living.

The Foreign Office in London, meanwhile, said it had received a request from Ecuador to help investigate the source of the bug and was "considering it".

Ecuadoran foreign minister Ricardo Patino said a microphone had been found during a routine search of the embassy ahead of his visit to London last month.

He said it was found in the office of ambassador Ana Alban and accused the Surveillance Group private investigations firm of installing it.

But Timothy Young, the British group's chief executive, said the allegation was "completely untrue".

"The Surveillance Group do not and have never been engaged in any activities of this nature," he said in a statement.

"We have not been contacted by any member of the Ecuadoran government and our first notification about this incident was via the press this morning. This is a wholly untrue assertion."

A spokesman for the Foreign Office refused to comment on whether there was a microphone in the embassy, saying it was a matter for the Ecuadoran government.

But he said: "We've received a request for assistance from the Ecuadoran government and are in the process of considering it."

Assange has been holed up at the embassy for a year after claiming asylum from Ecuador to avoid extradition to Sweden, where he is wanted for questioning over allegations of sexual assault against two women.

His case has caused tensions between Ecuador and Britain, which has refused to grant Assange safe passage out of the building, leaving him stranded.